Concretes and other related materials such as mortars, grouts and renders are typically formed by combining an aggregate material, such as sand and gravel, with a cementitious binder (cement). The most common cement used in the world today is Ordinary Portland cement (OPC). OPC is a finely-ground material containing at least two thirds by mass calcium silicate phases, with the majority of the remainder being made of aluminium, iron and magnesium based phases. When a mixture of OPC and an aggregate is further combined with water, a hydration reaction occurs and the mixture solidifies.
OPC has many benefits as a binder in concretes and related materials. Concretes produced using OPC are quick to set and cure to a high compressive strength. The raw materials for the manufacture of OPC are readily available and the cement itself is relatively cheap. Other cementitious materials, such as pozzolans or blastfurnace slags, may produce structures with final strength or environmental durability, but the setting and curing of such materials tends to be inferior in comparison with OPC based materials. Thus, conventional cement compositions comprise a proportion of OPC even if other cementitious materials are also used.
In recent years, the environmental impact of various industrial processes has become a great global concern. The production of OPC is a highly energy intensive process that involves various raw materials being heated in a kiln to temperatures greater than 1500° C., cooled, and then ground to a fine powder. It is estimated that about one tonne of carbon dioxide is released, as a result of chemical reactions that occur during heating and due to the combustion of fuels (1.6 GJ/tonne), for every tonne of OPC produced.
One cementitious material often used to replace a portion of OPC in concretes is ground granulated blastfurnace slag (GGBS). GGBS may be described as a non-OPC latently-hydraulically-active material. When iron ore has the iron taken out of it in a blast furnace, the non-metallic product consisting essentially of silicates and alumino-silicates of calcium becomes available to form a cementitious binding material. In the production of iron, a blast furnace is continuously charged from the top with iron oxide (ore pellets, sinter etc) and a fluxing stone comprising of limestone and dolomite together with coking fuel. Two products are obtained from the furnace; molten iron that collects as a pool in the bottom of the furnace and liquid iron-blast-furnace slag floating on the pool of iron. Both products are periodically tapped from the furnace at temperatures of about 1500° C.
To maximise the hydraulic potential of GGBS, molten slag must be chilled rapidly as it leaves the blast furnace. Rapid quenching or chilling minimises crystallisation and converts the molten slag into fine glassy aggregate-sized granules having dimensions generally smaller than 5 mm. These granules of slag are then ground to a fine powder to form GGBS. As GGBS is a by-product of the iron industry, it has a much lower carbon footprint than OPC (i.e. 0.055 tonnes CO2 per tonne of GGBS vs. ˜1 tonne of CO2 per tonne of OPC). Thus, the amount of carbon dioxide released per tonne of concrete can be reduced if a proportion of GGBS is used in conjunction with OPC as a binder. Typically, GGBS cannot replace greater than 70% of the weight of OPC to form a viable binder for a concrete or mortar. It would be desirable to eliminate the OPC altogether, but GGBS requires an activator in order to function as a hydraulic material.
The use of GGBS as a cementitious material has been known for many years and appears to date back to 1774 when Loriot made a mortar using GGBS in combination with slaked lime. The slaked lime was used as an activator. While cementitious binders formed from GGBS activated with lime may have useful properties, the set times and strength gain times are longer compared with an OPC equivalent.
The use of GGBS in conjunction with chemical stimulants such as alkalis is therefore known, but the skilled person's understanding is that such concretes have limitations in relation to setting times, strength and temperature response. This has prevented their widespread adoption. Concretes are specified and used according to established national standards, codes etc. Whilst such documents may acknowledge GGBS-based concretes they are not at the forefront of use. OPC concretes are in widespread use, are competitive and functionally adequate.
The skilled person's current understanding, or prejudice, is that the higher the GGBS content the more limited the mechanical properties of the resulting concretes, even using OPC as the activator for GGBS. Further, concretes have to satisfy certain standards e.g., in the UK, BS 4246 where minimum strengths against time have to be achieved. Thus, despite its environmental impact, OPC continues to be used.